1. Field of the Invention
This invention relates generally to semiconductor devices. More specifically, the invention relates to the formation of submicron metal contacts on substrates.
2. Description of the Prior Art
The art of forming images for the production of microelectric devices is well known. In this regard, photoresist compositions are widely used image-forming compositions for microelectronic device manufacturing processes. Generally, in these processes a thin coating of a radiation sensitive photoresist composition is first applied to a substrate material. The coated substrate is then treated to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The coated surface of the substrate is next subjected to an imagewise exposure to actinic radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used in microlithographic processes. After imagewise exposure, the coated substrate is contacted with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
There are two general categories of photoresist compositions--negative working and positive working photoresists. When negative working photoresist compositions are exposed imagewise to radiation, the areas exposed to radiation become less soluble to a developer solution while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative working resist with a developer causes removal of the non-exposed areas of the resist coating thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. When positive working photoresist compositions are exposed imagewise to radiation, those areas exposed to the radiation become more soluble to the developer solution while unexposed areas remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive working photoresist with the developer causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. A desired portion of the underlying substrate surface remains uncovered. Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics.
Imaging processes may be additive or subtractive in nature. Subtractive processes entail an etching away of material using dry plasma, a chemical solution, or an ion beam. In a subtractive process a substrate is coated with a resist and the resist layer is then imagewise exposed to radiation in order to degrade the resist in the exposed area. The resist is next immersed in a solvent, which dissolves away the exposed region, leaving the desired image. The resist layer then acts as a protective mask for the subsequent etching away of the material in the layer to be patterned. Remaining portions of the resist layer are then stripped away in a strong solvent, leaving the desired image. Additive processes are those where material is deposited after the resist has been patterned. In an additive or so called "lift-off" process, a metal is deposited after resist patterning and then the resist is stripped off, leaving metal in the open areas of the resist. In such a process, a substrate is coated with a resist layer which is then exposed and developed to dissolve away the exposed image areas. A metal or other material to be patterned is deposited on top of the resist layer, such that the metal or other material adheres to the substrate in the patterned regions. The resist layer is then removed and the excess metal sheared off leaving only a metal contact attached to the substrate.
One major drawback with the additive process, however, occurs when shearing the metal during lift-off in the final step. Using the conventional approach, excess metal located on top of the photoresist layer and along the side walls of the cavities created in the photoresist layer may not cleanly separate. In some cases, the excess metal may carry with it metal which was intended to be a part of the metal contact. Additionally, the excess metal may shear off such that a portion of the unneeded metal remains connected to the metal contact. In either case the metal contact may be unusable, thus rendering the entire device worthless and greatly increasing manufacturing costs.
It would therefore be highly desirable to have a means to provide a clean discontinuity of the deposited metal layer. While many skilled in the art have devised processing means to provide for a clean discontinuity, these prior art techniques add complexity to the fabrication process by requiring additional steps such as multiple electron beam radiation exposure. See, for example, U.S. Pat. No. 5,658,469. In general, this additional complexity tends to degrade overall process yields by adding a further source of variability. U.S. Pat. No. 5,468,595, which is incorporated herein by reference, shows an electron beam exposure technique wherein a photoresist is exposed at a controlled level, however, it does not describe use of a reflected electron beam bouncing upward, effectively re-exposing a part of the photoresist layer.
As part of a solution to the foregoing problems, a process has been found whereby the cavities of a photoresist image are expanded to include notches or wedge-shaped voids. These notches may then be filled with metal along with the cavities of the photoresist layer to form conductive attachments to the metal contacts. This results in metal contacts being more securely attached to the substrate. It has been found that during the shearing or metal lift-off step, the metal contacts remain substantially undamaged as a result of a stronger attachment to the substrate.